Several large studies have demonstrated a clear increase in mortality and cardiovascular events with sleeping fewer hours than needed. For instance, an analysis of the 70,000 female participant Nurses' Health Study demonstrated 83% increased (relative risk) likelihood of cardiovascular events with 5 hours sleep per night, 30% increased risk with 6 hours. Several other studies have made similar observations.

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Imagine missing your mortgage payment for a couple of months. Think your banker will just say, "That's okay. Just pick up where you left off?" Fat chance. Same with sleep: Accumulate a sleep debt, and you pay the price in one form or another. If you don't pay the "debt" back with sleep, you will pay with health.

But here's a peculiar discrepancy: Psychologists and sleep experts have for years advised us that the ideal quantity of sleep for optimal mental function is 9½ hours per night. This is the quantity of sleep that mimics the sleep habits of other primates (chimpanzees, gorillas, bonobo apes) and increases mental acuity, memory, vigilance, and reduces reaction time. Observations in people who are put in situations deprived of day/night triggers trend towards this amount of sleep, as well as observations of people in less time-stressed cultures. Thus, sleep experts have concluded that humans do best by obtaining 9½ hours of sleep per night.

But epidemiologic studies of sleep behavior, like the Nurses' Health Study, showed that sleeping more than 9 hours per night increased cardiovascular events and mortality!

Unfortunately, epidemiologic studies cannot make direct association and establish causation. In other words, people who sleep 9½ hours or more per night experience greater likelihood of cardiovascular events - but does the extra sleep cause the increased risk? Or do people who sleep 9½ hours or more per night do so because they are depressed or have some other unique lifestyle, health, or genetic factor that is associated with both greater sleep and cardiovascular risk?

The answers are not yet available. One thing is clear: Shortchanging sleep and getting less than 7 hours per night not only reduces your mental performance, but also can increase cardiovascular risk, diabetes, and can make you fat!